1. Field of the Invention
The present invention relates to a method for the gas sealing of a phosphoric acid-type fuel cell, and more particularly to an improvement of the gas sealing at the periphery of the electrode base and separator which constitute the fuel cell.
2. Description of the Prior Art
As known well, the phosphoric acid-type fuel cell is a device for generating electric power by the electrochemical reaction of an externally supplied fuel and oxidizing agent. Basically, it has two electrodes separated from each other by phosphoric acid which is an electrolyte and it operates at a temperature of 150 220.degree.-220.degree. C. under normal pressure to 10 atm.
For a phosphoric acid-type fuel cell, it is absolutely necessary and important to prevent the leakage of reaction gases inside and outside the cell. The leakage and mixing of reaction gases might lead to a serious accident. Effective and reliable tight sealing is important especially at the periphery of the electrode base and separator constituting the fuel cell.
One of the gas seal methods heretofore employed for the periphery of the electrode having the catalyst layer is wet sealing which is based on the capillary action of the electrolyte itself. This method is disclosed in Japanese Patent Publication No. 152/1983 (U.S. Pat. No. 3,867,206). According to this method, the periphery of the electrode base is filled with the electrolyte by the capillary action, taking advantage of the fact that the electrode base having the catalyst layer, which is used in a phosphoric acid-type fuel cell, is made of a gas-permeable porous carbon sheet. The periphery of the electrode base is filled with a filler such as tantalum, graphite, and polyarylsulfone to render it hydrophilic. The filler is intended to permit the periphery of the electrode to retain the electrolyte as much as desired. These fillers are not affected by the electrolyte. Silicon carbide (SiC) is a new filler which is in general use recently.
A disadvantage of the conventional wet seal is that it forfeits its sealing power at the wet sealing portion to a great extent when phosphoric acid is short and there occurs a pressure difference. Under such a situation, the phosphoric acid held by the filler easily flows out because it is highly fluid and mobile at the operating temperature and the operating pressure.
In order to overcome this disadvantage, the present applicant proposed a new sealing method. (See Japanese Patent Laid-open No. 13572/1986.) According to this method, the periphery of the electrode base is filled with a gel of phosphate compound to improve the sealing performance at the periphery of the electrode base. The phosphate compound enhances the capillary action and hence wettability on account of its powerful affinity for phosphoric acid. This method, however, does not eliminate the disadvantage that the gas sealing power at the wet seal part greatly decreases when phosphoric acid is short. In addition, this method involves a new problem that the phosphate compound in the wet seal part gradually dissolves in phosphoric acid and flows out into inner reservoir and outer reservoir, so that the phosphate compound finally disappears, with the result that the gas sealing performance becomes poor.
On the other hand, attempts have been made to apply the wet sealing to the periphery of the separator. An example is disclosed in Japanese Patent Laid-open No. 83360/1984. The application of wet sealing to the periphery of the porous reservoir plate having no catalyst layer is also disclosed in Japanese Pat. Application No. 210322/1986, "Fuel cell", filed by the present applicant on Sept. 4, 1986. In either cases, it was impossible to prevent the decrease of sealing power at the wet seal which occurs when phosphoric acid is short, so long as gas sealing resorts to the capillary action of liquid phosphoric acid.